1. Field of the Invention
This invention relates to a pulse counter circuit and a displacement measuring device employing this pulse counter circuit. The present invention are ideally usable, for example, on incremental-type rotary encoders or linear encoders.
2. Related Background Art
Conventionally, on magnetic or optical incremantal-type rotary encoders or linear encoders, displacement amounts of movable scales, shift directions, and others are detected by using two types of sinusoidal wave signals (2-phase sinusoidal wave signals) having mutually different phases and obtainable from two sensors. On linear encoders, for instance, it is known that signal insertion circuits are adapted to improve the resolution of position detection of linear scales. From the above-mentioned 2-phase sinusoidal wave signals, a plural number of sinusoidal wave signals mutually possessing phase differences are generated and based on these sinusoidal wave signals, a plural number of divided pulse signals are formed.
FIG. 1 is a block diagram showing an example signal insertion circuit. The components indicated with the numerals of 1 and 2 in this figure are input terminals. For example, input into these terminals are sinusoidal wave signals with mutually differing phases that are output from two sensors of a linear encoder. For instance, while signal "a" is input into input terminal 1, what is input into input terminal 2 is signal "b" possessing 90-degree phase difference from signal "a". Also, by inputting signal "a" into reverse circuit 3, signal "c" possessing 180-degree phase difference from signal "a" can be obtained. Then, by appropriately weighting these three types of sinusoidal signals "a", "b", and "c" with resistance and others, sinusoidal wave signals with optional angles (phase angles) are inserted. In this FIG. 1, the same values for resistance R are used to thereby obtain signals "d" and "e" with phase differences of 45 degrees and 135 degrees, respectively. These signals "d", "b", and "e" are converted to rectangular wave signals (binary signals) "f", "g", "h", and "i" by means of respectively corresponding comparator 4,5,6, and 7. The signals are then output in the form of a pulse train by direction discriminator 8. The pulses forming this pulse train are generally termed up-down pulses. Depending on phase relations between signals "a" and "b" (which is leading), these up-down pulses are divided into up pulse "j" or down pulse "k". In this manner, since original signal "a" (or "b") is subjected to multiple division, the resolution for angle detection can be upgraded. Shown in FIG. 2 are the waveform examples of signals "f" through "k" mentioned in connection with FIG. 1. In this case, pulse signal "j" ("k" during reverse rotation) is obtained with 8 division of one cycle of rectangular wave signal "f" (or "h") corresponding to input signal "a" (or "b"). By counting of these up-down pulses (divided pulses) "j" and "k", scale positions are detected.
However, when the linear scale shift speed is accelerated, frequencies of sinusoidal wave signals input into input terminals 1 and 2 are increased. It was found that as a result of this frequency rise, there occur cases where accurate measurement of linear scale displacement is not possible while the signal insertion circuit fails to follow those high-frequency signals and to correctly generate divided pulses.